Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55

To: mathgroup at smc.vnet.net

Subject: [mg45928] Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55

From: "Gunnar Lindenblatt" <Gunnar.Lindenblatt at pobox.com>

Date: Fri, 30 Jan 2004 04:15:54 -0500 (EST)

Sender: owner-wri-mathgroup at wolfram.com

Hi,
I just switched from another system to Mathematica
5. While all "complicated" operations (like importing Wave-Files, filtering,
parameter fitting, plotting) just work fine, I still have problems with the
"basics" like solving or even simplifying equations:
For example, to get the telegraph equation by self-induction and capacitive
coupling:
(One can solve this problem on the space of a postage stamp...)
In[1] := Remove["Global`*"]
In[2] := myEqn1 = -Dt[u,x] == r i + l Dt[i,t]
In[3] := myEqn2 = -Dt[i,x] == s u + c Dt[u,t]
Direct approach: Using "Solve"
In[4] := Solve[{myEqn1,myEqn2}, {Dt[u,{x,2}]}]
results an empty set of solutions:
{{}}
Second try: Using "Reduce"
In[5] := Reduce[{myEqn1,myEqn2}, {Dt[u,{x,2}]}]
results:
Reduce::nsmet: This system cannot be solved with the methods available to
Reduce.
Third try: Using "Eliminate"
In[6] := Eliminate[{myEqn1,myEqn2}, {Dt[i,x],Dt[i,t]}]
results:
True
That's fine! However, it does not really help me...
(By the way, the result should be:
Dt[u,{x,2}]== r s u + (r c + l s) Dt[u,t] + l c Dt[u,{t,2}])
Any ideas? -- Perhaps this problem is too simple for Mathematica, so it
rejects any help ;-)
- Gunnar
--
Gunnar Lindenblatt
e-mail: Gunnar.Lindenblatt at pobox.com